The present invention concerns a high current draining capacity micro-lightning arrester, that is a protective component for avoiding damage to electrical circuits or installations which may be subject to high electrical overloads.
It is necessary to provide general or special electrical installations with protective components to avoid detrimental effects of overload for which theses installations are not designed. These protective components are known as fuses, overvoltage protection tubes and lightning arresters. Their function is to stop the transmission of an overload dangerous for a given type of installation.
The application of this type of protective component is of particular importance in the case of telephone installations, circuits and exchanges. Indeed, telephone lines and exchanges are extremely vulnerable to lightning as well as to stresses caused by induced overvoltages or to overloads caused by accidental contact of a power transport line with a telephone line.
In order to meet the requirement for protecting telephone circuits and exchanges, it has become necessary to connect a protective component, commonly known as lightning arrester, between each line wire and ground. The word lightning arrester designates a device including, in particular, electrodes placed within an enclosure containing a gaseous atmosphere.
The required characteristics of this lightning arrester are to cause no loss under normal operating conditions of the line (that is to present infinite resistance to current flow) and, on the other hand, to withstand and conduct to ground any incidental overload (that is to present a low resistance which is always less than that of the circuit to be protected while having a current draining capacity above a predetermined threshold value).
Now, a discharge tube presents under a voltage, called starting voltage, almost infinite resistance. For voltages across it greater than the starting voltage, the tube discharges and presents a low resistance. Such a tube is able to withstand high overloads, provided that its structure is sufficiently rugged, and to conduct the overload towards ground. The starting voltage value is easily predetermined by adjusting the distance between the discharge electrodes. The current draining capacity is determined by the tube structure.
Safety and reliability of telephone lines and circuits require another characteristic of protective devices, such as lightning arresters; namely, the lightning arrester must form a short-circuit whenever it becomes defective. Indeed, if this requirement is not satisfied, nothing indicates failure of the protective component and the line would be destroyed by the first occurring overload. This can be avoided only by requiring the component to indicate its own failure. In this case, since the line no longer operates, it becomes necessary to correct its defective protection by changing the out of service component in order to restore the line to normal operation. It is for this reason that the component must present a dead short-circuit and stay in this state as soon as it can no longer perform its function, regardless of the cause of the failure. Grounding of the line makes it necessary to replace the defective protection component.
A lightning arrester meeting these requirements has been described in the French Patent Application Ser. No. 75 06524, filed on Mar. 3, 1975 and assigned to the present assignee, and issued on Mar. 5, 1979 as French Pat. No. 2,303,371. This lightning arrester includes, in particular, a sealed enclosure made of a metal which is a good electrical and thermal conductor, such as silver aluminum or copper filled with an inert atmosphere such as a mixture of rare gases; e.g., argon and helium at a pressure approximating 250 torr. This enclosure is closed by means of a plug made of insulating material capable of softening at a temperature lower than the softening or melting temperatures of the other parts of the lighting arrester. A first electrode traverses this plug and presents a discharge surface facing the discharge surface of a second electrode placed within the enclosure.
In normal operation, this lightning arrester acts as any discharge tube, that is, as long as the voltage across it remains lower than its starting voltage, it is at rest. When the voltage across it becomes equal to the starting voltage value determined by the gap between both electrodes, the discharge takes place. The lightning arrester can thus conduct a discharge nominal alternating current of about 5 A, during a well-determined time, generally at least equal to 50/I sec., I being the discharge current amplitude expressed in amperes, according to the recommendations of the CCITT (advice K12 CCITT--Geneva 1977).
When the operation becomes abnormal, in particular when the incident overload considerably exceeds the draining capacities provided by the manufacturer when producing the lightning arrester, an abnormal heating of the enclosure occurs. The internal temperature of the latter reaches and exceeds the plug softening temperature. As the internal pressure is smaller than the atmospheric pressure, the softened material is sucked inwards driving with it the first electrode. The resulting motion of the first electrode reduces the interelectrode gap ultimately to zero, whereupon the two electrodes are short-circuited.
This lightning arrester thus meets the characteristics required up to now. But the users of such a device, according to their needs, intend to modify their requirements and ask the manufacturers to provide a lightning arrester which can conduct a current of about 20 A. On the other hand, in case of abnormal operation, that is when the incident overload is substantially greater than the defined draining capacities, the lightning arrester must produce a short-circuit more quickly than previously, the temperature of the external enclosure of the lightning arrester having to stay within reasonable limits in order that the connection devices (for example, fuse-holders) not be damaged.
The lightning arrester described in the above-mentioned French patent and designed for responding to a draining capacity in conformity with the present requirements, does not meet the above-mentioned last criteria; i.e., the time for establishing the electrode short-circuit is too long for being acceptable under the future environment conditions of the lightning arrester (plug-in case in thermoplastic maaterial, for example). On the other hand, due to the fact that this time is important, the area of the lightning arrester external enclosure located near the interelectrode gap is substantially heated up. The temperature of this area then exceeds the allowed limits.